Two-way intercommunication system



c. E. MGcuLLoUGH 3,291,911

TWO-WAY INTERCOMMUNICATION SYSTEM 2 Sheets-Sheet l v Dec. 13, 1966 Filed April 1, 1965 Dec. 13, 1966 c. E. MCCULLOUGH TWO-WAY INTERCOMMUNICATION SYSTEM 2 Sheets-Sheet 2 Filed April l. 1963 I NVENTOR.

`mute) the second channel.

United States Patent O 3,291,911 TW O-WAY INTERCOMMUNICATION SYSTEM Charles E. McCullough, Austin, Tex., assignor to Kamari Aircraft Corporation, Bloomeld, Conn., a corporation of Connecticut Filed Apr. 1, 1963, Ser. No. 269,408 9 Claims. (Cl. 179-1) This invention relates to `a two-way intercommunication system for operation between two locations. More particularly, the invention relates to a novel two-way system which is capable of operation without harmful and annoying feedback. While the system is useful in a situation where background noise at the two locations is about equal, it is particularly useful in those situations where one of the microphone locations is subjected to a heavy concentration of audible background noise. The invention further relates to a completely automatic installation in which either station may transmit without the necessity for manual switching.

Harmful feedback is that characteristic high-pitched squeal yoftentimes heard over loud speakers at public gatherings and the like, which often proves very annoying to the person talking and is quite uncomfortable to the listening audience. Most prior art two-way systems have not been completely successful in two channel operations for a number of reasons, but particularly so because of two important reasons. First, most systems have used voice-operated switches or relays to activate the desired channel, which channel has previously been completely muted, but some delay is inherent. As la result, the beginning of speech occurs before operation of the switch can activate the communicating channel. This can severely limit the intelligibility of communications, especially in short, single-word transmissions.

A second big disadvantage of many systems is that they operate in such a way that the channel not in use, or the channel carrying the smallest signal is blocked or muted. This means that the person talking the loudest would dominate. Although such a system will definitely eliminate feedback, its operation is particularly unsatisfactory if one station is located in a noisy environment, such as a bank drive-in teller station, since the noise tends to keep the opposite channel (i.e., the channel having the microphone located in the low noise area) muted at `all times. This undesirable situation may occur in any system in which it is possible for either of the two channels to mute the other.

United States patents which are representative of the state of the art include: Hoard, 2,205,142; Jeanlin, 2,545,466; Jeanlin, 2,545,467; Levy, 2,545,476; Levy, 2,545,477; Hardy, 2,694,749; Jeanlin, 2,743,314; and Laurence, 2,878,314.

Briefly stated, the invention herein is a two-Way intercommunication system having two channels, one of which is arranged to be the dominating channel, but which dominating channel is partially muted, i.e., has reduced gain and/ or reduced high frequency response (preferably both) when it is not transmitting a voice communication. There is a second channel which is designed to operate at full gain and high frequency response at all times, except when there is communication through the dominating channel. Each of the channels has a microphone and a speaker with audio amplifier means connected therebetween. In addition, there are appropriate control circuits which normally and automatically partially mute the dominating channel but which automatically permit the dominating channel, when voi-ce communication originates at its microphone, to have increased gain and/ or high frequency response and thereby dominate (i.e., partially For convenience the condi- Patented Dec. 13, 1966 tion which exists when the dominating channel is operating at increased gain and/ or increased high frequency response will sometimes be referred to as increased sensitivity. These control cir-cuits are so arranged to also prevent harmful feedback.

It is an object of this invention to provide a two-Way intencommunication system which will operate automatically without clipping part of the speech or give the sensation of cutting on and olf.

It is another object of this invention to provide a system which is useful between two locations which have about equal background noise levels.

It is an object of this invention to provide a two-way communication system which can operate successfully where-one -of the locations has a high background noise level.

It is a further object of this invention to Provide a system which will operate automatically without harmful feedback.

Yet another object is to provide a system where neither channel is completely muted, but in which the gain of each may be reduced to avoid the dead mike sensation.

A still further object is to provide a communication system for automatic communication between two or more locations without harmful feedback, without the sensation of a dead microphone, and without clipping portions of speech transmissions.

These and other objects of the invention will be apparent by a fuller description of the invention by reference to the drawings wherein:

FIG. 1 is a block diagram showing one embodiment of the invention, and

FIG. 2 is a detailed diagram of the circuit arrangement of the system shown in FIG. l.

For convenience in designating the various parts in the figures and in presenting the description herein, the following abbreviations may from time to time be used: R-resistor Q-transistor CR-crystal diode or rectier C-capacitor Mic-microphone Spier-speaker Dr-driver PrA-pre-amplier PWA-power amplifier ARF-amplifier, rectifier, lter circuit CC-control circuit SJ-sum junction -l--positive voltage -negative voltage v A-indicates a com-mon connection with other elements so similarly designated B-indicates a common connection to a power source D (2h-dominating channel S C11-second channel RC-resistance capacitance More specifically, the invention to Ibe described overcomes the foregoing short-comings of the prior art with respect to apparent switching or dead microphone effect, by simply partially muting, i.e., limiting the gain and/or the high frequency response (but preferably both), of one of the channels instead of ymuting it completely when it is not in use. Thus, limiting either the overall .gain or the high frequency response (preferably both) will limit the gain over the frequency range where harmful feedba-ck ordinarily occurs, but will still allow enough signal to pass at the lower (voice) frequencies at all times so that the sensation of switching is less or non-apparent.

The problem wit-h -respect to high audible background noise at one of the stations is overcome by allowing only one channel, i.e., the dominating channel, to have control of the gain of the opposite channel, i.e., the second channel, rather than permitting either channel to control the other. This desired result is accomplished by a circuit means inthe form of a negative feedback system between the dominating channel and the second channel, whereby when the dominating channel is operating at increased gain and/or high frequency response, i.e., increased sensitivity, which would ordinarily be caused by a communication originating at the microphone of the dominating channel, there is a negative feedback to the second channel which perfor-ms two functions.

First, this negative feedback :system partially mutes, i.e., reduces the gain and/ or high frequency response of the second channel, whenever the dominating ch-annel is in use, i.e., when communication is originating at the microphone of the dominating channel. Second, it eliminates the possibility of regenerative feedback by automatically partially muting, i.e., reducing the gain Jand/or high frequency response of the second channel whenever feedback starts to occur. Thus, the desired mode of operation is obtained when the input to the second channel is from the station in the noisy environment, since noise into the second channel cannot block or mute the dominating channel.

yReference to the drawings `will explain the preferred operation of the system in greater detail. FIG. 1 shows a first channel or dominating channel as D Ch 18 and a second channel as S Ch 19. D Ch 18 has a first microphone, Mic 20 and a first speaker, Spkr 24. Mic 20 and Spkr 24 are connected by audio amplifier means in the form of a pre-amplifier indicated as PrA 21, a driver -as Dr 22, and a power amplifier as PWA 23.

S Ch 19 is lsimilarly arranged with Mic 20', PrA 21', Dr 22', PWA 23', and Spkr 24. If one of the locations to be communicated between is relatively high inaudible background noise, then Spkr 24 and Mic 20' would be located as shown at high-noise location 11, and Mic 20 and Spkr 24' would be positioned as shown at low-noise location 10.

It is obvious that if `the communication system consisted of no addition-al control circuits, sound could travel from Mic 20, through D Ch 18 to Spkr 24 to adjacent Mic 20" and similarly 'back toMic 20, which would result in harmful regenerative feedback. With the additional control circuits which are provided, this cannot occur.'

Thus, there is provided appropriate control circuits, which in the preferred form utilize transistors, but vacuum tubes could Ibe employed. Each of the Q numbers designated in the various components shown on FIG. 1 correspond to Itransistors shown in FIG. 2, and the single letters R likewise refer to resistors.

As shown in FIG. 2, power is supplied in the form of a common direct current to those portions of the circuit indicated by B. Both PrA 21 and PrA 21' are connected to a decoupling filter compris-ing R 55 and C 133 by leads (not shown) 'joined at points indicated by the letter A in FIG. 2.

'D Ch 18 is automatically controlled so as to -be normally, partially muted, i.e., to operate at reduced gain and/or reduced high frequency response, when no communication is originating at Mic 20, by a first means in the form of an electrical control circuit shown as CC 27, which 'takes some of ythe signal from Dr 22, i.e., places an extra electrical load on Dr 22, which in the form shown, is done by varying the RC load thereon, as will lbe explained hereinafter.

A second means is also provided for cutting off or deactivating CC 27 when communication is originating at Mic 20, but not when communication is originating only from Mic 20', which means takes the form of -a second electrical control circuit comprising ARF 25, ARF 42, SJ 29, and lead 32 to CC 27.

A third means for automatically reducing the gain of S Ch 19 when D Ch 18 is operating at increased gain conveniently takes the form of a third electrical circuit 4. comprising ARF 28 and CC 34, which varies t-he gain of Dr 22', as will be explained hereinafter.

Operation of the system will now be described in five phases as follows:

(l) No signal present at either Mic 20 or Mic 20',

(2) Input signal only to D Ch 18 at Mic 20,

(3) Input signal only to S Ch 19 at Mic 20',

(4) Noise input only Ito S Ch 19 at Mic 20', and

(5) Input signal to both channels at Mic 20, 20l simultaneously. y

(1) No signal present at either Mic 20 or Mic 20': When no signal is present at either Mic 20 or Mic 20', which condition will be referred to as normal operation, S Ch 19 will be operating at full gain and fidelity and/ or frequency response. However, D Ch 18 will be partially muted, i.e., operating at reduced gain and reduced high frequency response, because of the operation of CC 27. Bias 26, in the form of a negative voltage crea-ted by R 77 is placed on the base of Q 120, which places a parallel RC load (in the form of C 148 and R '78) on Dr 22, which then reduces the value of the signal PWA 23. Since there is no signal at either Mic 20 or Mic 20', CC 27 is controlled only by bias 26. Regenerative feedback cannot occur since any increased sensitivity, i.e., increased gain and/or high frequency response in D Ch 18 would produce a signal that would be transmi-tted to S Ch 19 by a third electrical circuit means in the form of ARF 28, Where the signal would be amplified, rectified and filtered to produce a positive voltage which would automatically, through CC 34, reduce the gain of S Ch 19 by varying the bias on Dr 22.

ARF 28 and CC 34 are so arranged that they are not responsive to the reduced gain and low frequency responses communicated by D Ch 18 during normal operation, but are responsive to increased gain -and/or higher frequencies, i.e., increased sensitivity, Which occurs when communication originates at Mic 20. Thus, a positive voltage is applied to Dr 22' only when D Ch 18 is operating at increased sensitivity.

(2) Input signal only to D Ch 18 at Mic 20: Since the gain and high-frequency response of D Ch 18 are normally reduced as long as no input signal is present at Mic 20, CC 27 must increase these to a higher level before a signal can be transmit-ted at full volume and fidelity. This is accomplished by a first rectifier or connecting circuit in the form of ARF 25, which takes the signal at PrA 21 caused by communication originating at Mic 20, and amplifies, rectifies and filters the signal to produce a positive signal which is applied to SJ 29. SJ 29 is in the form of a potentiometer which is adjustable to be in balance when receiving equal and opposite voltage contributions from ARF 25 and ARF 42, which balance would only occur when communication is origin-ating only at Mic 20. Therefore, when a positive voltage is supplied to Si 29 from ARF 25, as in thev present case, it will not be balanced by a voltage from ARF 42, hence a positive voltage is supplied to CC 27 via lead 32, which voltage is arranged -to overcome bias 26, at which point Q can no longer conduct 'and the RC load on Dr 22 is removed, permitting D Ch 18 to operate at increased gain and high frequency response. When communication is no longer originating at Mic 20, D Ch 18 will return to normal operation.

The values of C 148 and R 78 which place the RC load on Dr 22 are selected to control the gain and to control the high frequency response of D Ch 18, but yalso to provide enough residual gain and low-frequency response so that the sensation of switching or turning on and off is not noticeable.

Regenerative feedback cannot occur since, as described above, when the gain and high frequency response of D Ch 18 is increased, a positive voltage is transmitted to Dr 22 via ARF 28, lead 33, CC 34, and lead 35. The resulting decrease in negative bias on Dr 22 decreases the gain of S Ch 19 to prevent feedback.

(3) Input signal only to S Ch 19 at Mic 20': Since S Ch 19 normally operates at full gain and fidelity, any input signal applied to S Ch 19 via Mic 20 could pass directly to Spkr 24'. However, if it were not for a second rectifier or connecting circuit in the form of ARF 42, the sound from Spkr 24' would enter Mic 20 and cut off CC 27, thereby increasing the gain on D Ch 1S, which would permit a signal to go to ARF 28 and be applied to Dr 22', with the resulting undesirable effect being a reduction in the gain of S Ch 19, while it should be at normal full gain to carry the communication originating at Mic 20'.

To overcome this difficulty, the signal from PWA 23' is amplified, rectified and filtered by ARF 42, to provide a negative voltage which is applied to SI 29, which is a variable resistor, as described above, and which may be adjusted for the optimum relative positive and negative contributions from ARF 25 and ARF 42, and thus be in balance when communication is originating only at Mic 20. As the output signal from S Ch 19 increases, the output voltages from ARF 25 and ARF 42 increase in approximately the same proportion, hence there will be no output voltage from SI 29 to CC 27 and therefore the sound originating from Spkr 24 will have no effect on the gain of S Ch 19.

(4) Noise only input to S Ch 19 at Mic 20': The operation of the system with noise input to S Ch 19 at Mic 20' is essentially the same as for a communication signal into S Ch 19 as described in phase- (3) above. It is important to the operation of the system that SJ 29 be balanced, i.e., properly adjusted so that the voltage from ARF 25 into CC 27 resulting from sound originating in Spkr 24' is exactly cancelled at SI 29 by the opposite polarity voltage from ARF 42. Thus, whenever the sound into Mic 20 is from Spkr 24', CC 27 maintains the RC load on Dr 22. However, additional sound into Mic 20, in the form of a communication originatingat Mic 20 will produce an increased output voltage from ARF 25 which is not cancelled by ARF 42, and CC 27 will then increase the gain and high frequency response of D Ch 18, which will then be the dominating channel and will reduce the gain of S Ch 19. Hence, the noise into Mic 20 is audible in Spkr 24', but does not interfere with normal automatic operation of the two-way system.

(5) Input signal to both Mic 20 and Mic 20': When there is communication originating at Mic 20 and Mic 20 simultaneously, D Ch 18 will, of course, dominate just as described above in phase (4) when noise is originating at Mic 20. The gain of S Ch 19 will be reduced until communication ceases at Mic 20.

Thus, there is provided a two-way intercommunication system which works automatically without impairing speech transmission and without the dead microphone effect, which operates wtihout `troublesome feedback, and which is useful particularly where one location is subject to high background noise.

There are, of course, lmany variations which are possible with this invention. Variable resistors R 98 and R 100 are provided so as -to establish the desired volume controls. The values of the various components would vary from installation to installation, depending on the particular circumstances of each. For example, Q 120, R 77, C 148 and R 78 are selected so as to place the desired load on Dr 22. Also, C 147, C 146, R 79 and R 99 are selected to provide the desired speed at which S] 29 is to function.

As shown in FIG. 2, CC 34 reduces only the gain of S Ch 19. It could be -arranged to also limit the high frequency response of S Ch 19 by the addition of another transistor, in much the same manner as CC 27. For convenience, the condition which exists when S Ch 19 is operating at reduced gain and/or reduced high frequency response sometimes will be referred to as partially muted.

Other amplifier means, such as amplifiers utilizing electronic tubes, could be used as well as the transistor amplifiers shown.

The system is useful in a variety of situations where two-way communication is necessary between two locations separated by a sound barrier. It is particularly useful where one station has a high audible noise level since the system operates without harmful feedback as described above.

The positive voltage applied to Dr 22' by CC 34 when D Ch 1S is operating at increased gain could, of course, be taken from SI 19, which provides a positive voltage only when there is communication originating at Mic 20. However, the design shown is preferred because of -the more uniform signal received from vPWA 23 via ARF 28.

The system could be operated as a multi-channel system by providing D Ch 18 with an additional speaker a-t a third location and S Ch 19 with an additional microphone located at the third location, thus providing an additional location which could be in communication with the dominating channel.

The novel arrangement of the system and the positive control -on feedback has presented the art with a highly successful two-way intercommunication system. There are actually two controls on feedback in that the electrical circuit formed by ARF 28 is one means of control and the operation of SI 29, when properly adjusted, acts as another control. Further, there is no noticeable squeal coming over the speakers when not transmitting communications.

Modifications may be made in the invention as particularly described without departing from the scope of the invention. Accordingly, the foregoing description is to be construed illustratively only and is not to be construed as a limitation upon the invention as defined inthe following claims.

I claim:

1. ln `an intercommunication system capable of twoway voice communication without regenerative feedback, the combination comprising:

a dominating channel having a first microphone located at a first location, a first speaker located at a second location, and audio amplifier means connected therebetween,

a second channel normally operating at full gain and having a first microphone located at said second 1ocation, a second speaker located at said first location, Iand audio amplifier means connected therebetween,

a first electrical circuit means automatically, partially muting said dominating channel when there is no communication originating at said first microphone,

a second electrical circuit means automatically deactivating said first means during the time when communication is originating at said first microphone, and

a third electricalcircuit Imeans automatically, partially muting said second channel when said dominating channel is operating at increased sensitivity, muting of said second channel being responsive only to cornmunication originating at said first microphone.

2. In an intercommunication system capable of twoway voice communication without regenerative feedback for operation between a llow audible noise location and a relatively high audible noise location, the combination comprising:

a dominating channel having a first microphone located in said low noise location and a first speaker located in said high noise location and audio amplifier means connected therebetween,

a second channel normally operating at full gain and having a second microphone located in said high noise location and a second speaker located in said low noise location and audio amplifier means connected therebetween,

a first electrical means automatically and normally maintaining said dominating channel in a partially muted state,

a second electrical means automatically deactivating said first means during the time when there is communication originating at said first microphone,

a third electrical means automatically reducing the gain lof said second channel when said dominating channel is operating at increased sensitivity, and

a fourth electrical means connecting said lsecond channel originating at said first microphone, a second electrical circuit connected between said domisaid high noise location and an audio amplifier means connected therebetween, Y

a second channel having a second microphone located in said high noise location,'a second speaker located with Said second electrical means for transmitting a in said low noise location, and audio amplifier means Signal from said second channel to said second elecconnected therebetween, trical means for automatically keeping said second a first control circuit connected to said dominating chanelectrical means from deactivating said first electrical nel automatically and normally reducing the gain and means until communication originates at said first high frequency response of said dominating channel, microphone. lo a second control circuit automatically transmitting .a 3. In an intercommunication system capable of twofirst signal from said dominating channel to said first way Voice communication without regenerative feedback, 'control circuit when communication is originating at the combination comprising; said first microphone, which first signal is adapted to a dominating channel having a first microphone located Cnt Off Said first Control circuit,

et a first location, a first Speaker located et a Second a third control circuit transmitting a second signal from location, and audio amplifier means connected there- Said dominating Channel t0 Said SeCOnd Channel when between, said dominating channel is operating at increased gain a second channel norm-ally operating at full gain and and high frequency reSPonSe, Which Seid Second Signiil having a second microphone located at said second controle the `goin of Said Second channel, "the gain of location, a second speaker located at said first loca- Said Second channel being controlled 'only in reSPonSe cation, and audio amplifier means connected thereto eSignal originnting in Said dominating channel, and between, a third signal from said second channel to said second e fu-St electrical circuit connected to Said dominating control circuit to negative transmission of signal from channel and automatically, ,partially muting said said second control circuit to said first control circuit dominating lchannel when there is no communication until communication originates et Said St microphone. 6. In an intercommunication system capable of twoway voice communication without harmful regenerative feedback for operation between a low audible noise location and a relatively high audible noise location, the cornbination comprising:

nating channel and said first circuit and automatically deactivating said first circuit during the time when communication is originating at said rst microphone, a third electrical circuit connecting said dominating channel and said second channel and partially unuting said second channel when said dominating channel is operating at increased sensitivity, muting of said second channel, muting of said second channel being responsive only to communication originating at said first microphone, and a fourth electrical means cona dominating channel having a first microphone located 1n said low noise location, a first speaker located in said high noise location, and audio amplifier means ond channel being responsive only to said dominating connected therebetween. .Channel at increased .sensitivity a second chanuelnormally operating at full gain and 4. A communication system for communicating behdelity and having a Second microphone located in tween two locations separated by a sound barrier com- Seid high noiSe location, a Second Speaker located in prising; said low noise location, and audio amplifier means a dominating channel having a rst microphone located connected therebetween,

at a first 1oeation, a {i1-5t Speaker located at a Sooa first control circuit connected to said dominating ond location, and laudio amplifier means connected channel and automatically and normally Partially therebetween, muting said dominating channel, a second channel normally operating at full gain and a Second control circuit comprising:

fidelity and having a Second microphone located at a first rectifier circuit connecting said dominating said second location, a second speaker located at said channel and a Sum junction, first location, and audio amplifier means connected a Second rectifier circuit connecting Said Second therebetween, channel and said sum junction, said sum junca first electrical means connected to Said dominating lion being ndlnstedio he in balance When COmchannel and automatically and normally partially 5o mhnlcatlon 1S originating only at Seid Second muting said dominating channel, microphone, and a second electrical means connecting said dominating a circuit means between Said Sum jonction and channel and said first electrical means Iand auto- Said hrst Control circuit and automntically transunatically deactivating said first electrical means when mitting n Signal therebetween 'only When Corncommunication is originating at said first microphone, munication 1S originating at Said rSt IniCrO- a third electrical means connecting said dominating Phone, Which Said Signal cuts Off Said first Conchannel and said second channel and automatically trol circuit, transmitting a signal to said second channel when o third control circuit automatlcally transmitting a said dominating channel is operating at increased Signal from Seid .domineting Channel t0 Said Second sensitivity, which said signal partially mutes said sec- 6o channel When Said dominating channel iS 'operating at increased sensitivity, which said signal partially mutes said second channel. 7. In an intercommunication system capable of twoway communication without harmful regenerative feedback for operation between .a low audible noise location and a relatively high audible noise location, the combination comprising:

a dominating channel having a first microphone located in said low noise location, connected with a preamplifier, a driver, a power amplifier, and a first speaker, which said first speaker is located in said high noise location,

a second channel having a second microphone located in said high noise location, connected with a second pre-amplifier, a second driver, a second power amplinecting said second channel with said second electrical means for automatically keeping said second electrical means from deactivating said first electrical means until communication originates at said first microphone.

5. Inan intercommunication system capable of twoway voice communication without harmful regenerative .o feedback for operation between a low audible noise level location and a relatively high audible noise level location the combination comprising:

a dominating channel having a first microphone located in said low noise location, a first speaker located in 9 fier and a second speaker, which said second speaker is located in said low noise location,

a first control circuit connected to said first driver and automatically placing an extra electrical load thereon, which load partially mutes said dominating channel,

a second control circuit comprising:

a first rectifier circuit connecting said first preamplifier and a sum junction,

a second rectifier circuit connecting said second channel and said sum junction, said sum junction being adjusted to be in balance when communication is originating only at said second microphone, and

a circuit means automatically transmitting a signal from said sum junction to said first control circuit when said sum junction is unbalanced by a communication originating at said first microphone, which said signal is adapted to remove said load from said first driver, and

a third control circuit automatically transmitting a signal from said dominating channel to said second channel when said dominating channel is operating at increased sensitivity, which said signal controls the gain of said second channel.

8. A communication system for communicating between two locations separated by ia sound barrier comprising:

a dominating ch-annel having a first microphone located at a first location, a first speaker located at a second location, and audio amplifier means connected therebetween,

a second channel normally operating at full gain and having a second microphone located at said second location, a second speaker :located at said first location, and audio amplifier means connected therebetween,

a first electrical control circuit connected to said dominating channel and automatically and normally causing said domin-ating channel to be partially muted,

a second electrical control circuit comprising:

a first connecting electrical circuit connecting said dominating channel and ia sum junction and transmitting a first signal therebetween when communication is originating at said iirst microphone,

a second connecting electrical circuit connecting said second channel and said sum junction and transmitting a second signa-i originating at said second microphone, said sum junction being adjusted to be in balance when communication is originating only at said second microphone, and

a lead connecting said sum junction and said first control circuit for transmitting a signal therebetween, which said signal cuts ott said rst control circuit,

a third electrical control circuit connecting said dominating channel and said second channel for automatically communicating a signal therebetween, which said signal partially mutes said second channel.

9. A communication system for communicating between at least two locations separated by a sound barrier 10 comprising:

a dominating channel and a second channel, each of said channels comprising a microphone, audio amplifier means and a speaker connected together,

a first electrical control circuit connected to said dominating channel and vautomatically and partially muting said dominating channel when communication is not originating at said microphone connected to said dominating channel,

a second electrical control circuit comprising a first amplifier, rectifier, filter circuit connecting said dominating channel and a sum junction, and transmitting a signal to said sum junction when said dominating channel is receiving an audio signal,

a second amplifier, rectifier, filter circuit connecting said second channel and said sum junction, and transmitting a signal to said sum junction when said second channel is transmitting a comrnunication, said sum junction being adjusted to be in balance when communication is originating at the microphone of said second chanan electrical :lead transmitting a signal from said sum junction to said first electrical control circuit When said sum junction is unbalanced, which signal deactivates said first electrical control circuit when communication is originating at the microphone of said dominating channel, and

a third electrical control circuit comprising an amplifier, a rectifier, and a filter, which said third electrical control circuit transmits a signal from said dominating channel to said second channel when said dominating channel is operating at increased sensitivity, which said signal partially mutes said second channel.

References Cited by the Examiner UNITED STATES PATENTS 3,113,181 12/1963 Soderbaum 179170.8 X

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,291 ,911 December 13 1966 Charles E. McCullough or appears in the above numbered pat- It is hereby certified that err rs Patent should read as ent requiring correction and that the said Lette corrected below.

Column 6 line 9 for "19" read 29 H column 8 line E2, for "a third signal" read M a fourth control circuit automatically transmitting a third signal Signed and sealed this 22nd day of April 1969.

(SEAL) Attest:

EDWARD I. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. IN AN INTERCOMMUNICATION SYSTEM CAPABLE OF TWOWAY VOICE COMMUNICATION WITHOUT REGENERATIVE FEEDBACK, THE COMBINATION COMPRISING: A DOMINATING CHANNEL HAVING A FIRST MICROPHONE LOCATED AT A FIRST LOCATION, A FIRST SPEAKER LOCATED AT A SECOND LOCATION, AND AUDIO AMPLIFIER MEANS CONNECTED THEREBETWEEN, A SECOND CHANNEL NORMALLY OPERATING AT FULL GAIN AND HAVING A FIRST MICROPHONE LOCATED AT SAID SECOND LOCATION, A SECOND SPEAKER LOCATED AT SAID FIRST LOCATION, AND AUDIO AMPLIFIER MEANS CONNECTED THEREBETWEEN, A FIRST ELECTRICAL CIRCUIT MEANS AUTOMATICALLY, PARTIALLY MUTING SAID DOMINATING CHANNEL WHEN THERE IS NO COMMUNICATION ORIGINATING AT SAID FIRST MICROPHONE, A SECOND ELECTRICAL CIRCUIT MEANS AUTOMATICALLY DEACTIVATING SAID FIRST MEANS DURING THE TIME WHEN COMMUNICATION IS ORIGINATING AT SAID FIRST MICROPHONE, AND A THIRD ELECTRICAL CIRCUIT MEANS AUTOMATICALLY, PARTIALLY MUTING SAID SECOND CHANNEL WHEN SAID DOMINATING CHANNEL IS OPERATING AT INCREASED SENSITIVITY, MUTING OF SAID SECOND CHANNEL BEING RESPONSIVE ONLY TO COMMUNICATION ORIGINATING AT SAID FIRST MICROPHONE. 